This invention is directed to a breathable multilayer film having a core layer and one or two adjacent skin layers. The core layer contains a mixture of thermoplastic polymer and particulate filler, and is rendered breathable to moisture (water) vapor when the film is stretch-thinned following coextrusion with the skin layer or layers. The skin layer or layers prevent filler particles in the core layer from building up at the die lip during the coextrusion. Cracks, voids, tears and/or breaks are formed in the skin layer or layers during stretching to increase the breathability of the overall film.
Breathable stretch-thinned films, and laminates including a breathable film and a nonwoven web, are used in diaper backings, other personal care products, and medical garments. For example, U.S. Pat. No. 5,695,868 issued to McCormack, discloses a breathable film and a breathable, cloth-like film/nonwoven composite which includes the film thermally bonded to a fibrous polyolefin nonwoven web.
Breathable layered films have also been prepared for use in personal care products, diapers and medical related items such as surgical drapes and gowns, as is shown in U.S. Pat. No. 6,075,179 to McCormack et al. In this patent, the film includes a core layer coextruded with one or two adjacent skin layers. The core layer, which accounts for most of the film thickness, can be formed from a mixture of a thermoplastic polymer and optionally including a quantity of a particulate filler. When the film is stretched subsequent to coextrusion, voids form around the filler particles in the core layer. It is believed that the voids allow for concentration diffusion through the porous film. This diffusion is what causes the film to have water vapor breathability. The skin layer or layers account for a minor percentage of the film thickness, and may together constitute less than 15% of the film thickness. As described in the ""179 patent, the skin layers can be prepared from a thermoplastic polymer or a blend of polymers. The skin layers serve two main purposes. One purpose is to partially shield the die lips from the core layer during coextrusion, so that the filler particles and polymer from the core layer do not excessively build up at the die lips. Another purpose is to serve as a bonding layer, which facilitates bonding of the coextruded film to a nonwoven web.
One disadvantage of skin layers is that they tend to reduce the moisture vapor breathability of the overall film, since skin layer tends to block the pathways created by the pores in the core layer. The thicker the skin layers, the greater the reduction in breathability. Optimization of skin layer thickness requires providing enough skin layer thickness to control die build-up during extrusion and provide bonding to a nonwoven web, yet the skin layer thickness should not be so great that overall breathability is substantially impaired. This can be a difficult balance to achieve, especially in view of the trend toward more breathable films and laminates.
In U.S. patent application Ser. No. 09/385,988, filed Aug. 27, 1999, by McCormack et al., which is commonly assigned to the Assignee of the present invention, a multilayer breathable film is disclosed. The multilayer laminate comprises a core and at least one skin layer, wherein the layers are coextruded. The core is formed from a mixture of at least one thermoplastic polymer and at least one particulate filler. When the film is stretched subsequent to coextrusion, voids form around the filler particles in the core layer. It is believed that the voids allow for concentration diffusion through the porous film. This diffusion is what causes the film to have water vapor breathability. The skin layer comprises a mixture of two incompatible polymers, wherein the incompatibility of the polymer results in the formation of cracks and/or tears when the laminate film is stretched. There is no suggestion in the ""988 application to add a compatibilizer to the incompatible polymers of the skin layer or layers. Breathable films of the ""988 application and laminates including the film are disclosed to be useful in a wide variety of person care absorbent articles, medical garments and other similar products.
The present invention is directed to a breathable, multi-layered stretch-thinned film including a core layer and one or two adjacent skin layers. The core layer is made from a mixture of one or more thermoplastic polymers and a particulate filler, such that voids are formed around the filler particles when the film is stretched. The skin layer or layers are formed from a polymer blend comprising at least two incompatible polymers and a compatibilizer. During the coextrusion of the multilayer film, the skin layer(s) maintain integrity, yet, the skin layer(s) crack, break, tear or form voids when the film is stretched.
It has been discovered that the addition of the compatibilizer allows for morphology control of the skin layer during formation, which in turn improves the film consistency of the skin layer.
As a result of the present invention, the skin layers can be made thick enough to adequately prevent extrusion die build-up caused by the particulate filler in the core layer. The skin layers can also be as thick as necessary to provide adequate bonding of the film to a substrate, such as a nonwoven web. Because the skin layers crack, break, tear or form voids during stretching of the film, the skin layers do not significantly impair the moisture vapor breathability of the overall film. In addition, the breathability of the resulting film is less dependent upon the thickness of the skin layers and more dependent on the intrinsic properties of the composition of the skin layer.
Further, the skin of the stretched-thin film has an improved processability and appearance after stretching. The addition of the compatibilizer reduces the presence of xe2x80x9cbarringxe2x80x9d in the skin layer. Barring is the non-uniform stretching of a film wherein portions of area of the film stretch while other areas do not stretch or stretch at a different point in time with the rest of the film.
The present invention also includes a breathable laminate comprising a nonwoven web and the breathable stretched-thinned film. The resulting laminate is breathable and can be used in a wide variety of applications including personal care products, protective covers and medical garments. It has been discovered that the breathable laminates of the present invention have improved bonding characteristics between the breathable stretch-thinned film and the nonwoven web.
The present invention also includes a method of preparing a stretch-thinned breathable film. The method includes the steps of providing a core layer composition containing one or more thermoplastic polymers and a particulate filler, providing a skin layer composition for one or both sides of the core layer, coextruding the core layer composition and skin layer composition together through a die to form a multilayer film having a core layer and at least one skin layer, stretching the film in at least one direction after it leaves the die, and cracking or tearing the skin layer during stretching of the film. An important feature of the method is that the skin layer is cracked or has voids formed during stretching, after it leaves the die, instead of being cracked at the die during extrusion.
Finally, another object of the present invention is to provide a method of preparing a laminate of a nonwoven web and the breathable stretched-thinned film.